Insight into the nature of active redox sites in Ru-containing hydroxyapatite by DRIFT spectroscopy

The excellent selectivity and activity of ruthenium-hydroxyapatite {RuHAp, HAp = Ca10(PO4)6(OH)2} in the aerobic oxidation of alcohols has been attributed to isolated, monomeric Ru3+ species as the active sites. Here we reinvestigated these catalysts by a DRIFT study of CO adsorption and by electron microscopy (STEM) combined with EDX analysis. The study revealed the presence of two major species: isolated Ru3+ ions and hydrated Ru3+-oxide particles. The latter structure has high redox activity; it is easily oxidized with oxygen to RuO2 and reduced by hydrogen or CO at ambient conditions. The amount of this phase in the catalysts correlates well with the alcohol oxidation activity in the liquid phase at close to ambient conditions. In contrast, the redox activity of isolated Ru3+ ions is significantly lower (or they might be completely inactive). These species may correspond to adsorbed Ru3+ ions, ion-exchanged Ru3+ at the outer surface of HAp, or Ru3+ ions in Ru-phosphate formed by a dissolution–redeposition process from HAp. Our final conclusion is that in RuHAp, the real active sites are small hydrated Ru3+-oxide nanoparticles (1–2 nm) located on the surface of HAp. Application of a low RuCl3 concentration and a short contact time between HAp and the aqueous RuCl3 solution during catalyst preparation help maximize the fraction of the Ru3+-oxide phase and thus the catalytic activity.